Composite propellants containing activated carbon



United States Patent .0

2,994,598 COMPOSITE PROPELLANTS CONTAINING ACTIVATED CARBON Myron L. Dickey, Whittier, Califi, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Mar. 23, 1959, Ser. No. 801,392

' 17 Claims. (Cl. 52.5)

This invention relates to the preparation of propellants of improved burning rate.

In recent years, considerable work has been directed toward the development of solid propellants suitable for use in rocket motors. Rockets employing solid propellant fuels are extensively used by the military and have also achieved considerable importance in commercial operations. For example, solid rocket propellants are utilized in missiles, projectiles, and rockets to assist planes in takeofl? or to furnished an extra surge of power in flight. A solid propellant to be satisfactory for its intended use should possess certain desirable physical characteristics, for example, the propellant should have a high tensile strength and a high modulus of elasticity.

Recently, superior solid propellant materials have been discovered which comprise a solid oxidant, such as ammonium nitrate or ammonium perchlorate, and a rubbery material, such as a copolymer of butadiene and a vinylpyridine or other substituted heterocyclic nitrogen base compound, which after incorporation is cured by a quaternization reaction or a vulcanization reaction. Solid propellant mixtures of this nature and a process for their production are disclosed and claimed in copending application, Serial No. 284,447, filed April 25, 1952, by W. B. Reynolds and J. E. Pritchard.

In the production of solid propellants it is ordinarily desirable that said propellants have excellent mechanical properties such as high elongation and high tensile strength. However, for some special purpose propellants the mechanical properties are of secondary importance. This is frequently the situation where .a high impulse propeHant is desired for certain specific uses. A high impulse propellant can be defined as one having a high burning rate and which thus produces large volumes of gas in relatively short periods of time. Such high impulse propellants'are frequently desired for use in relatively small missiles such as air-to-air missiles, ground-to-air missiles and stationary gas generators and where the physical properties of the propellant itself are of secondary importance.

- High impulse propellants utilize burning rate catalysts which increase the rate or burning of the propellant composition. A burning rate catalyst which is frequently compounded into the propellant comprises carbon black. Carbon black is particularly desirable in propellants utilizing a rubbery polymer as a component of the binder. This invention is concerned with propellants which include carbon black as one of-its components and with a method of increasing the activity of the carbon black as a burning rate catalyst therein.

Accordingly, it is an object of the invention to provide a high impulse propellant of improved burning rate. Another object is to provide a method of increasing the burning rate of a propellant which contains carbon black. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad embodiment of the invention comprises a propellant containing a binder, an oxidizer, and carbon black Patented Aug. 1, 1961 Propellants compounded in accordance with the invention comprise a binder in the form of a rubbery copoly-, mer together with a plasticizer therefor, a solid oxidant, and a burning rate catalytic material comprising carbon black. The oxidizer content of the propellant composition is usually in the range of 50 to percent by weight and the binder content is usually in the range of 10 to 50 weight percent of the propellant. The amount of carbon black is usually in the range of 0.5 to 30 weight percent of the binder.

The rubbery polymers employed as binders in the solid propellant compositions of this invention are copolymers of conjuated dienes with polymerizable heterocyclic nitro gen bases of the pyridine series. These copolymers can vary in consistency from very soft rubbers, i.e., materials which are soft at room temperature but will show retraction when relaxed, to those having a Mooney value ('ML 4) up to 100. The rubber copolymers most frequently preferred have Mooney values in the range between 10 and 40. They may be prepared by any polymerization methods known to the art, 'e.g., mass or emulsion polymerization. One convenient'method for preparing these copolymers is by emulsion polymerization at temperatures in the range between 0 and 140 Recipes such as the iron pyrophosphate-hydroperoxide, either sugar-free or containing sugar, the sulfoxylate, and the persulfate recipes are among those which are applicable. It is advantageous to polymerize to high conversion as the unreacted vinylpyridine monomer is difiicult to remove by stripping. Said copolymers can have carbon black incorporated therein an amount within, the range of O to 30 parts of carbon black per parts of copolymer. The conjugated dienes employed are those containing from 4 to 8 carbon atoms per molecule and include 1,3- butadiene, isoprene, 2-methyl-1,3-butadiene, 2,3-dimethyl- 1,3-butadiene, Z-methoxybutadiene, Z-phenylbutadiene, and the like. Various alkoxy, such as methoxy and ethoxy and cyano derivatives of these conjugated dienes, are also applicable. Thus, other dienes, such as phenylbutadiene, 2,3-dimethyl-l,2-hexadiene, 2-methoxy-3 ethylbutadiene, Z-ethoxy-S-ethyl-1,3hexadiene, Z-cyanO-LB-butadiene, are also applicable in the preparation of the polymeric binders of this invention. Furthermore, instead of using a single conjugated diene, a mixture of conjugated dienes can be employed. Thus, a mixture of 1,3 butadiene and isoprene can be employed as the conjugated diene portion of the system.

The polymerizable heterocyclic nitrogen bases which are applicable for the production of the polymeric materials are those of the pyridine, quinoline, and isoquinoline series which are copolymerizable with a conjugate diene and contain one, and only one,

RI CH C rivatives are also applicable but the total number of carbon atoms in the groups attached to the carbon atoms of the heterocyclic nucleus should not be greater than 15 because the polymerization rate decreases somewhat with increasing size of the alkyl group. Compounds where the alkyl substituents are methyl and/or ethyl are available commercially.

These heterocyclic nitrogen bases have the formula R R R R R R R R where R is selected from the group consisting of hydrogen, alkyl, vinyl, alpha-methylvinyl, alkoxy, halo, hydroxy, cyano, aryloxy, aryl, and combinations of these groups such as haloalkyl, alkylaryl, hydroxyaryl, and the like; one and only one of said groups being selected from the group consisting of vinyl and alpha-methylvinyl; and the total number of carbon atoms in the nuclear substituted groups being not greater than 15. Examples of such compounds are 2-vinylpyridine; 2-vinyl-5-ethylpyridine; 2- methyl-S-vinylpyridine; 4-vinylpyridiue; 2,3,4-trimethyl-- vinylpyridine; 3,4,5,6-tetramethyl-2-vinylpyridine; 3-ethyl- 5-vinylpyridine; 2,6-diethyl-4-vinylpyridine; 2-isopropyl-4- nonyl-5-vinylpyridine; 2-methyl-5-undecyl-3-vinylpyridine; 2,4-dimethyl-S,6-dipentyl-3-vinylpyridine; 2-decyl-5-(alpha-methylvinyl)pyridine; 2-vinyl-3-methyl-5-ethylpyridine; 2-methoxy-4-chloro-6-vinylpyridine; 3-vinyl-5-ethoxypyridine; 2-vinyl-5,5-dichloropyridine; Z-(alpha-methylvinyl)-4-hydroxy-6-cyanopyridine; 2-vinyl-4-phenoxy-5- methylpyridine; 2-cyano-5-(alpha-methylvinyl)pyridine; 3-vinyl-5-phenylpyridine; 2-(para-methyl-phenyl)-3-vinyl- 4-methylpyridine; 3-vinyl-5-(hydroxyphenyl)-pyridine; 2- vinylquinoline; 2-vinyl-4-ethylquinoline; 3-vinyl-6,7-di-npropylquinoline; 2-methyl-5-nonyl-6-vinylquinoline; 4(alpha-methylvinyl)-8-dodecylquinoline; 3-vinylisoquinoline; l,6-dimethyl-3-vinyl-isoquinoline; 2-vinyl-4 benzylquinoline; 3-vinyl-5-chloroethylquinoline; 3-vinyl-5,6-dichlor0- isoquinoline; 2-vinyl-6-ethoxy-7-methylquinoline; 3-vinylfi-hydroxymethylquinoline; and the like.

The presently preferred plasticizer is a liquid polybutadiene prepared by mass polymerization in the presence of finely divided sodium as the catalyst according to the method of U.S. Patent to Crouch 2,631,175. Broadly, the plasticizers which can be used in accordance with this invention comprise liquid polymers prepared from conjugated diolefin hydrocarbons such as 1,3-butadiene and isoprene, the liquid polymers having a viscosity of 100 to 5000 Saybolt Furol seconds at 100 F. Polymers having a viscosity from about 1000 to about 2500 Saybolt =Furol seconds are presently preferred. These liquid polymers can be prepared by emulsion polymerization using large amounts of modifiers in accordance with the teaching of [Frolich et a1. 2,500,983, although they are preferably prepared by the method set forth in Crouch 2,631,175. The latter method comprises mass polymerization in the presence of finely divided alkali metal and/or alkali metal hydride such as sodium, potassium, lithium, sodium hydride, potassium hydride and lithium hydride. Polymers thus prepared contain no modifiers or viscosity controlling agent and they are also free of materials which would act as inhibitors such as antioxidants and shortstops.

Oxidants which are applicable in the solid propellant compositions of this invention include ammonium, alkali metal, and alkaline earth metal salts of nitric and perchloric acid. Ammonium nitrate is the presently preferred oxidant for use in the solid rocket fuels of this invention. Specific oxidants include ammonium perchlorate, sodium nitrate, potassium perchlorate, potassium nitrate, calcium nitrate, and barium perchlorate. Mixtures of oxidants are also applicable. In the preparation of the solid rocket propellant compositions, the oxidants are powdered to sizes preferably finer than 200 mesh.

Other burning rate catalysts may be employed in conjunction With the activated carbon black. Ammonium dichromate is the preferred burning rate catalyst to be added. Other burning rate catalysts which may be utilized include metal ferrocyanides and ferricyanides, ferric ferrocyanides, such as Prussian, Berlin, Hamburg, Chinese, Paris, and Milori blue, soluble ferric ferrocyanide, such as soluble Berlin or Prussian blue which contains potassium ferric ferrocyanide, and ferric ferrocyanide which has been treated with ammonia. Ferrous ferricyanide, Turnbulls blue is also applicable. A particularly effective burning rate catalyst is Milori blue which is a pigment similar to Prussian blue but having a red tint and is prepared by the oxidation of a paste of potassium ferrocyanide and ferrous sulfate. Other metal compounds such as nickel and copper ferrocyanides can also be employed. The amount of burning rate catalyst, other than carbon black, used in the propellant compositions of this invention is usually in the range 0 to 4 parts by weight based on the total weight of the propellant.

The activation of the carbon black in accordance with the invention comprises heating the same in admixture with a dilute aqueous solution of sulfuric, nitric, hydrochloric, acetic, or chloroacetic acid at a temperature in the range of about 100 to 250 C. for a period of at least 10 minutes and preferably for at least 30 minutes. Activation under superatmospheric pressure at higher temperatures in the above range shortens the time required for activation. A suitable method of activating the carbon black comprises heating the black suspended in the acid under total reflux for a period of one hour or more. The acid concentration may range from about 1 to 25 percent and the concentration of solids in the solution may vary over a broad range from merely enough liquid to completely wet the carbon black to a concentration as low as a few percent such as l to 10 percent of solids in the solution. Following the heat treatment of the carbon black, it may be incorporated in the binder in either wet or dry form. However, it is preferred to flash dry the carbon black following its recovery from the acid solution by filtering, or other method, and washing the black to remove the acid, before mixing the black with the binder.

To illustrate the invention, two propellants were made of identical compositions except for the activation of the carbon black in one and the use of unactivated carbon black in the other. A l,3-butadiene-2-methyl-5-vinylpyridine rubbery copolymer, prepared by emulsion polymerization at 41 F., was placed on a roll mill and worked for several minutes, after which Philblack E (a trademark of Phillips Petroleum Company, designating a super abrasion furnace black) was worked into the rubber. The black was previously activated by refluxing for several hours at 100 C. with 10 percent H in a 10' percent solids solution. The resulting rubber was taken off the rolls and placed in a Baker-Perkins mixer and worked until the power requirements increased. At this point plasticizer (polybutadiene) was added and working was continued until power requirements indicated that plasticization was completed. The dry ingredients were than added in four equal increments with all of the MgO, Milori blue, and ammonium dichromate being added in the last increment. After addition of each increment the mixer was operated until there was a sharp increase in power requirements, indicating that the mixing was completed. The same procedure was utilized in preparing a second propellant, excepting for the activation of the Philblack E which was omitted. The table below shows the composition of the two propellants in weight percents. Standard strands for burning rate tests were made from each of the mixes and tests were made on the burning rate and also to determine the pressure exponent. These data are also shown in the table.

The data clearly indicate that activation of the carbon black appreciably increases the burning rate of a propellant composition comprising the ingredients listed.

The disclosed compositions are formed into suitable grains by conventional means such as by extrusion or compression molding. The grains are cured at a temperature in the range of 70 to 250 F., preferably in the range of 140 to 200 F. The curing time must be long enough to produce the desired mechanical properties in the propellant and will range from around 3 hours to 28 days, depending upon the temperature. Higher temperatures require less curing time and vice versa.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A propellant consisting essentially of to 50 weight percent of a binder composed of a rubbery copolymer of a conjugated diene of from 4 to 8 carbon atoms per molecule with a heterocyclic nitrogen base and a polybutadiene plasticizer; from 50 to 90 weight percent of a solid inorganic oxidant consisting essentially of at least one nitrate of the group consisting of ammonium, alkali metal, and alkaline earth metal nitrates; and an amount of carbon black in the range of 0.5 to 30 weight percent of said binder, said black having been activated by heating same for a period of at least 10 minutes at a temperature in the range of about 100 to 250 C. in admixture with a dilute aqueous solution of an acid selected from the group consisting of sulfuric, nitric, hydrochloric, acetic, and chloroacetic.

2. The propellant of claim 1 wherein said oxidant consists essentially of ammonium nitrate.

3. The propellant of claim 2 including Milori blue and ammonium dichromate in total amount up to 4 weight percent of said propellant.

4. The propellant of claim 3 including magnesium oxide in an amount up to 4 weight percent of said propellant.

5. The propellant of claim 1 wherein said carbon black has been activated by refluxing with said acid.

6. The propellant of claim 1 wherein said acid is sulfuric.

7. The propellant of claim 1 wherein said acid is nitric.

8. The propellant of claim 1 wherein said acid is hydrochloric.

9. The propellant of claim 1 wherein said acid is acetic.

10. The propellant of claim 1 wherein said acid is chloroacetic.

11. A propellant consisting essentially of 10 to 50 weight percent of a binder composed of a rubbery copolymer of a conjugated diene of from 4 to 8 carbon atoms per molecule with a heterocyclic nitrogen base selected from the group consisting of the pyridine, quinoline, and isoquinoline series and their substituted derivatives having not more than 15 carbon atoms in the groups attached to the carbons of the heterocyclic nucleus and a plasticizer of polybutadiene; from 50 to weight percent of an oxidizer comprising at least one nitrate of the group ammonium' nitrate, alkali metal nitrate, and alkaline earth metal nitrate; and a burning rate catalyst consisting essentially of carbon black activated by heating in admixture with a dilute aqueous solution of an acid of the group sulfuric, nitric, hydrochloric, acetic, and chloroacetic for a period of at least 10 minutes at a temperature in the range of about to 250 C., said catalyst comprising from 0.5 to 30 weight percent of said binder.

12. The propellant of claim 11 wherein said diene is butadiene, said base is methylvinylpynidine, said plasticizer is polybutadiene, and said nitrate includes ammonium nitrate.

13. The propellant of claim 12 wherein said acid is sulfuric.

.14. In the preparation of a solid propellant composed of 10 to 50 weight percent of a binder composed of a rubbery copolymer of a conjugated diene of from 4 to 8 carbon atoms per molecule with a heterocyclic nitrogen base and a polybutadiene plasticizer, and from 50 to 90 weight percent of an inorganic oxidant comprising at least one nitrate of the group ammonium, alkali metal, and alkaline earth metal nitrates, the method of increasing the burning rate of said propellant which comprises activating carbon black by heating same for a period of at least 10 minutes at a temperature in the range of about 100 to 250 C. in admixture with a dilute aqueous solution of an acid selected from the group consisting of sulfuric, nitric, hydrochloric, acetic, and chloroacetic; and incorporating the activated carbon black in said propellant in an amount in the range of about 0.5 to 30 weight percent of the binder.

15. The method of claim 14 wherein said binder consists essentially of a copolymer of a conjugated diene with a heterocyclic nitrogen base and polybutadiene admixed with said carbon black.

16. The method of claim 15 wherein said oxidant comprises ammonium nitrate.

17. The method of claim 16 wherein said acid is sulfuric.

References Cited in the file of this patent UNITED STATES PATENTS Thomas Oct. 21, 1958 Fox Mar. 17, 1959 OTHER REFERENCES 

1. A PROPELLANT CONSISTING ESSENTIALLY OF 10 TO 50 WEIGHT PERCENT OF A BINDER COMPOSED OF A RUBBERY COPOLYMER OF A CONJUGATED DIENE OF FROM 4 TO 8 CARBON ATOMS PER MOLECULE WITH A HETEROCYCLIC NITROGEN BASE AND A POLYBUTADIENE PLASTICIZER, FROM 50 TO 90 WEIGHT PERCENT OF A SOLID INORGANIC OXIDANT CONSISTING ESSENTIALLY OF AT LEAST ONE NITRATE OF THE GROUP CONSISTING OF AMMONIUM, ALKALI METAL, AND ALKALINE EARTH METAL NITRATES, AND AN AMOUNT OF CARBON BLACK IN THE RANGE OF 0.5 TO 30 WEIGHT PERCENT OF SAID BINDER, SAID BLACK HAVING BEEN ACTIVATED BY HEATING SAME FOR A PERIOD OF AT LEAST 10 MINUTES AT A TEMPERATURE IN THE RANGE OF ABOUT 100 TO 250*C. IN ADMIXTURE WITH A DILUTE AQUEOUS SOLUTION OF AN ACID SELECTED FROM THE GROUP CONSISTING OF SULFURIC, NITRIC, HYDROCHLORIC, ACETIC, AND CHLOROACETIC. 